1. Field of the Invention
This invention relates to communications spacecraft, and more particularly to a low cost method and apparatus for satellite amplification of communication signals within different channels (satellite transponders) that are separate from each other in frequency.
2. Description of the Related Art
An increase in the RF power capability of high power satellite amplifiers along with an increase in the DC power capability of commercial satellites has led to the transmission of higher power satellite downlink beams for applications such as direct broadcast television. While this is very helpful when a large area is to be covered by a single beam and/or when the beam broadcast includes a large number of transponders, new applications are emerging in the satellite communications field that utilize smaller "spot" beams rather than large composite patterns. Such an application is described in a co-pending patent application by John L. Norin et al. filed on the same day as this application, Ser. No. 60/062,004. "Non-Uniform Multi-Beam Satellite Communications System and Method", in which direct local television service is provided through a non-uniform pattern of local spot beams.
The standard technique for transmitting multiple signal channels from a single satellite is to provide a separate high power amplifier dedicated to each channel. For example, 32 active amplifiers would be used for a frequency plan covering 32 channels. This works well for large coverage areas, or smaller coverage areas with high power requirements. However, when smaller and lower power spot beams are to be transmitted, this results in excess output power capacity and weight when amplifiers with standard power ratings are used, and the amplifiers will be required to operate at less than efficient levels. If an attempt is made to match the power rating of each amplifier with the signal power within its respective channel by providing a variety of different amplifier sizes, the result is an increase in cost (particularly if non-standard amplifier sizes are required) and a requirement for extra redundancy rings to protect from unit failures, all of which increase the satellite cost.
High power amplifiers typically weigh on the order of 5 kg each, so a requirement for 32 separate amplifiers adds significantly to the total payload weight. At present each amplifier costs on the order of $250,000 and adds roughly another $150,000 to launch costs. The direct and indirect costs of amplifiers for a satellite with 32 downlink channels can thus exceed $10,000,000.
A method used on occasion for channel amplification has been to route multiple channels that are adjacent in frequency through a single amplifier. This is used for low power applications, such as the input receiver of a payload, but is rarely useful for high power amplification due to the intermodulation distortion that results when the amplifier is operated near saturation. To reduce the intermodulation components to acceptable levels the amplifier may have to be operated closer to its linear region, resulting in reduced DC power consumption efficiency. Even if the possibility of intermodulation distortion is accepted along with a more expensive DC power consumption, the individual channels cannot be routed to different downlink antennas as is frequently desirable, particularly for a spot beam application.